C/NOFS is a collaborative US minisatellite mission undertaken within the Space Test Program (STP) of DoD (Department of Defense), AFRL (Air Force Research Laboratory) and SMC/TEL (Space and Missile Command Test and Evaluation Directorate). Project participants include NASA, NRL (Naval Research Laboratory), UTD (University of Texas at Dallas), and the Aerospace Corporation. The overall goal of C/NOFS is to forecast for the first time the presence of ionospheric irregularities in Earth's equatorial region that adversely impact satellite communication and navigation systems (like GPS). 1)2)3)4)

C/NOFS is a prototype operational system designed to demonstrate the monitoring and forecasting of the ionospheric scintillation in real-time and on a global scale. Ambient ionospheric scintillations are caused by naturally occurring irregularities and lead to the degradation of satellite communications signals and of the GPS navigation signals (primarily in the UHF and L-band range). Scintillations are responsible for decreased satellite-to-ground message throughput and for delayed signal acquisition. The objective is also to detect conditions that could lead to scintillation in particular regions and thus provide outage forecasts (3-6 hours before its onset) for affected communication frequencies and potential GPS signal degradation.

C/NOFS is a global forecast system rather than a local "nowcast" system which observes the ionospheric scintillation currently occurring. The mission includes three critical elements:

1) A spaceborne system consisting of a proven sensor package to provide data for global, real-time specification, and a forecast capability

2) A series of regional ground networks that augment the spaceborne sensors for real-time, high-resolution coverage in theater

3) A forecasting/decision aid software package that produces tailored space environmental forecasts and warnings in the form of outage maps.

During the C/NOFS mission, the estimates are expected to extend to 24 hours from the time of the data collection.

Table 1: Major science topics and questions to be addressed by the C/NOFS mission

Figure 1: Schematic of the formation in latitude variation of ionization density in the equatorial F region, known as the equatorial anomaly or the Appleton anomaly (image credit: AFRL)

Legend to Figure 1: The diagram illustrates that during daytime the eastward dynamo electric field from the E region maps along the magnetic field to F-region heights above the magnetic equator. The plasma moves upward due to E x B drift and then diffuses along the magnetic field to form two crests with maximum ionization density near ±15º magnetic latitude and minimum ionization at the magnetic equator.

The C/NOFS spacecraft is funded/provided by DoD as well as the launch and the first year of on-orbit operations. The minisatellite was built, tested and integrated by Spectrum Astro Inc. of Gilbert, AZ, a division of General Dynamics C4 Systems (prime contractor). The S/C structure is of modular design with standard interfaces to allow for independent bus and payload development and reconfiguration; it features a smooth shape with all external surfaces conductively and electrically bonded together to minimize surface-charge potential differences. In addition, an EMI-tight Faraday cage is utilized with unobstructed science FOV (Field of View). 5)

The S/C has dimensions of 3.40 m in length and 1.16 m in diameter, with a bus mass of 314 kg and 70 kg of payload mass. The orbit average power consumption is 160 W for the bus and 64 W for the payload in "survey mode" and 197 W for the bus and 63 W for the payload in the "forecast or operational mode." The mission design life is 1 year with a goal of 3 years.

The mission data will be made available to the scientific community. AFRL (program office at Hanscom Air Force Base, Bedford, MA) is responsible for the instrument payload, payload integration and test, model development, data center operations, and data product generation and distribution. The CINDI science data processing and distribution is conducted at UTD. SMC/TEL of Kirtland AFB is providing spacecraft/mission operations.

The C/NOFS operational project is divided into three mission phases:

• Initial vehicle and experiment checkout phase - about 1 month

• Survey mode (Data collection and ionospheric model verification and validation) - 9 months). In this mode, the requirement for real-time data is lifted and a full complement of instruments will collect data to be used in the development of specification and forecast models.

• Forecast mode (Scintillation Forecasting) - 3 months required, 27 month goal. During this period (also referred to as "operational mode"), key instruments will be operated on a more limited duty cycle. The data from the instruments will be transmitted in real-time to the Data Center where it will be processed to provide global ionospheric scintillation specification and forecasts for the use of the US Air Force. The data collected by the satellite will be available for the use of the scientific community during all phases of the mission.

Mission status:

• Dec. 15, 2015: The C/NOFS (Communication/Navigation Outage Forecasting System) satellite burned up in Earth's atmosphere during a planned reentry on Nov. 28, 2015, leaving behind a treasure trove of data about a part of the space environment that's difficult to study. The unique set of sustained observations from C/NOFS will greatly improve models currently used to predict satellite trajectories, orbital drag and uncontrolled reentry. 9)

- Launched on April 16, 2008, C/NOFS studied a region high above in our atmosphere called the ionosphere, a layer of electrically charged particles created by ultra-violet radiation from the sun. This layer lies some 60 to 100 km above the Earth's surface, where it interacts and co-mingles with the neutral particles of the tenuous upper atmosphere. The upper atmosphere and ionosphere change constantly in response to forces from above and below, including explosions on the sun, intense upper atmosphere winds, and dynamic electric field changes. In addition to interfering with satellite orbits, such changes can produce turbulence in the ionosphere that cause what's known as scintillations, which interfere with radio wave navigation and communication systems, especially at low latitudes near the equator.

- During its last 13 months of operations, as its orbit decayed and it spiraled into lower altitudes and eventual reentry into Earth's atmosphere, the C/NOFS satellite captured a unique set of comprehensive observations as it traveled through the very space environment that can directly cause premature orbital decay. Such regions have rarely been studied directly for extended periods of time, because orbits in this denser region of the atmosphere are not sustainable long-term without on board propulsion.

- "One thing we learned clearly from C/NOFS is just how hard it is to predict the precise time and location of re-entry," said Cassandra Fesen, principal investigator for C/NOFS at AFRL (Air Force Research Laboratory) at the Kirtland Air Force Base in Albuquerque, New Mexico.

- The C/NOFS data at these lower altitudes show that the upper atmosphere and ionosphere react strongly to even small changes in near-Earth space, said Rod Heelis, principal investigator at the UT-Dallas for NASA's CINDI (Coupled Ion-Neutral Dynamics Investigation) instrument suite on board the satellite. "The neutral atmosphere responds very dramatically to quite small energy inputs," said Heelis. "Even though the energy is put in at high latitudes – closer to the poles – the reaction at lower latitudes, near the equator, is significant."

- Robert Pfaff, project scientist for CINDI at NASA/GSFC (Goddard Space Flight Center) in Greenbelt, Maryland, and principal investigator for another C/NOFS instrument, the VEFI (Vector Electric Field Investigation), is studying observations that speak to one of the original goals of the C/NOFS program: Why does the low latitude ionosphere at night become so turbulent that it can wreak havoc on communications and navigation radio signals?

- Developing the capability to predict such space weather disturbances has been a long-standing goal of the Air Force Research Laboratory. The C/NOFS low altitude observations were critical to form a complete picture of these disturbances, as the satellite ventured to the possible root of the largest ionospheric upheavals — those that emanate from the bottom ledge of the ionosphere at night. The observations revealed the presence of strong shears in the horizontal ionosphere motions at the base of the ionosphere, places where charged particles flow by each other in opposite directions. C/NOFS observed shears and undulations along this boundary. Such shears and undulations — spotted throughout the nighttime, equatorial ionosphere — are believed to be the source of large-scale instabilities that ultimately drive the detrimental scintillations.

• July 2015: The CINDI instrument onboard the C/NOFS satellite is approved by NASA to continue planning against the current budget guidelines through the duration of the mission, expected to end sometime in 2016 due to reentry, and through mission closeout. Any changes to the guidelines will be handled through the budget formulation process. Due to satellite reentry, the CINDI mission will not be invited to the 2017 Heliophysics Senior Review. 10)

- CINDI is a NASA Mission of Opportunity that flies on C/NOFS; it consists of two instruments: the IVM (Ion Velocity Meter) that measures ion density, temperature, composition and velocity, and the NWM (Neutral Wind Meter) that measures neutral atmosphere pressure and wind. CINDI produced excellent wind and density results while C/NOFS perigee was near 350 km. These combined with the continuing downward progression of CINDI with altitude confirm that there will be more extensive NWM data collection and analysis during this proposal period. As part of the operational C/NOFS program, the Air Force supports CINDI instrument operations, data access and distribution, and satellite orbit and position data.

• June 2015: The CINDI extended mission plan is well conceived and addresses compelling science topics that are uniquely facilitated by the measurements that CINDI will make during the next year as C/NOFS descends prior to reentry into the Earth's atmosphere. Additional data and operations are clearly required to accomplish the proposed science objectives. CINDI will continue to be the only mission providing coincident information on the dynamic state of the ionosphere and thermosphere as the solar cycle declines. This makes it an important data set for achieving system science of the effect of the Sun and magnetosphere on the Earth atmosphere. 11)

- Among CINDI accomplishments during its initial extended mission phase is the demonstration that longitude variations in the ionosphere and thermosphere are well aligned, suggesting that ion drag influences the distribution of the neutral species. The CINDI team also showed significant thermospheric density responses to variable solar flux and magnetic activity.

- CINDI's in situ measurements of the ionosphere and thermosphere constitute unique contributions to the HSO. They provide perspectives on how the geospace system responds to variable solar and geomagnetic forcing as well as the impacts of the lower atmosphere on the thermosphere-ionosphere. Any interpretation of CINDI data requires a detailed description of the external drivers. CINDI represents an important component of the HSO (Heliophysics System Observatory), enabling the inclusion of the connected ionosphere and thermosphere system in a coherent study of the geospace environment.

- The state of the C/NOFS satellite and CINDI IDM instrument performance is excellent and the NWM performance is improving as the orbit allows for viable thermospheric measurements near perigee. All indicators suggest that satellite and instrument performance will continue to be excellent until the satellite reentry, which is anticipated to occur sometime in 2016.

• On April 16, 2013, the C/NOFS spacecraft of AFRL celebrated its 5th anniversary on orbit. Originally scheduled to collect data for two years, the satellite has continued to provide crucial observations for five years running. CINDI, a NASA instrument in the Explorer Program, has had the opportunity to watch as the sun has increased in activity, and with its direct measurements of how neutral and charged particles interact, the instruments continue to help improve predictions of just when disturbances in the ionosphere will be at their worst. 12)

One of the early observations by CINDI was of the top of the ionosphere layer, which is dominated by hydrogen ions near dawn. The middle layer of the area is dominated by oxygen ions. In 2008, CINDI found that the transition region, where there is an equal number of both particles, was located about 370 miles up (~ 595 km), much closer to Earth than expected.

Over five years of watching, this oxygen/hydrogen transition region has now moved up in space to over 430 miles (~692 km) in altitude, providing an indicator of how Earth's atmosphere swells and expands in response to increased energy coming in from the sun.

Figure 5: Schematic view of GPS radio waves travelling through a disturbed layer of Earth's electrically charged atmosphere, the ionosphere, they can be disrupted (image credit: NASA)

• The C/NOFS spacecraft and its payload are operating nominally in 2012 (4th year on orbit). 13)

• NASA's VEFI (Vector Electric Field Instrument) aboard C/NOFS has detected Schumann resonances from space. This comes as a surprise, since current models of Schumann resonances predict these waves should be caged at a lower altitude, between the ground and a layer of Earth's atmosphere called the ionosphere. 14)

The researchers didn't expect to observe these resonances in space, but it turns out that energy is leaking out and this opens up many other possibilities to study our planet from above. The waves created by lightning do not look like the up and down waves of the ocean, but they still oscillate with regions of greater energy and lesser energy. These waves remain trapped inside an atmospheric ceiling created by the lower edge of the "ionosphere" – a part of the atmosphere filled with charged particles, which begins about 100 km up into the sky. In this case, the sweet spot for resonance requires the wave to be as long (or twice, three times as long, etc) as the circumference of Earth. This is an ELF (Extremely Low Frequency) wave that can be as low as 8 Hz. As this wave flows around Earth, it hits itself again at the perfect spot such that the crests and troughs are aligned. Voila, waves acting in resonance with each other to pump up the original signal.

Figure 6: Waves created by lightning flashes – here shown in blue, green, and red – circle around Earth, creating something called Schumann resonance. These waves can be used to study the nature of the atmosphere they travel through (image credit: NASA, Fernando Simoes)

Schumann resonances had been predicted in 1952; they were first measured reliably in the early 1960s. Since then, scientists have discovered that variations in the resonances correspond to changes in the seasons, solar activity, activity in Earth's magnetic environment, in water aerosols in the atmosphere, and other Earth-bound phenomena. 16)

C/NOFS measured the Schumann resonances at its orbital altitude range of 400-800 km. While models suggest that the resonances should be trapped under the ionosphere, it is not unheard of that energy can leak through. So the project team at GSFC began looking for waves of the correct, very low frequency in the observations from VEFI – an instrument built at NASA/GSFC with high enough sensitivity to spot these very faint waves. And the team was rewarded. They found the resonance showing up in almost every orbit C/NOFS made around Earth, which added up to some 10,000 examples.

Detection of these Schumann resonances in space requires, at the very least, an adjustment of the basic models to incorporate a "leaky" boundary at the bottom of the ionosphere. But detecting Schumann resonance from above also provides a tool to better understand the Earth-ionosphere cavity that surrounds Earth.

• Very interesting findings in the C/NOFS mission correspond to results related to low solar activity and EUV flux. A few of these findings are listed below: 17)

Figure 7: Average nighttime DN/N from May 2008 to March 2010, measured from C/NOFS (image credit: AFRL, Ref. 17)

- Due to the low solar EUV levels, the altitude extent of the ionosphere was significantly smaller than reference models would predict for these levels of solar activity. The height at which 50% of the ions are from atomic oxygen was as low as 450 km at night and rose to only 850 km during the day.

- Deep ambient plasma depletions were observed at dawn. Plasma irregularities were often embedded within them. Their frequencies strongly depended on longitude and season. Coincident polar satellite passes showed that these depletions were narrow in the zonal direction (~15°) but very wide in the meridional direction (~50°). Using electric field measurements from C/NOFS as inputs, the assimilative physics-based ionospheric model (PBMOD) successfully reproduced these density depletions, thus confirming that they were caused by strong upward ion convection drifts.

- Plasma density irregularities were mostly observed after midnight and at dawn, instead of just after sunset as they had been expected. The absence of equatorial plasma bubbles just after sunset was consistent with the E- field measurements: the pre-reversal enhancements in the E-field were almost never seen during this solar minimum. Similar effects observed in DMSP data confirmed that irregularities were present in the dawn side of the orbit rather than in the dusk side.

In conclusion, C/NOFS provided unusual observations of the ionosphere and its coupling to the lower atmosphere during this last very low solar minimum (Ref. 17).

• The C/NOFS spacecraft and its payload are operating nominally in 2010.

• The C/NOFS spacecraft and its payload are operating nominally in 2009. 18)

• December 2008: Observations made by the CINDI instrument have shown that the boundary between the Earth's upper atmosphere and space has moved to extraordinarily low altitudes. 19)20)21)

• CINDI's first discovery was, however, that the ionosphere was not where it had been expected to be. During the first months of CINDI operations the transition between the ionosphere and space was found to be at about 420 km altitude during the nighttime, barely rising above 800 km during the day. These altitudes were extraordinarily low compared with the more typical values of 640 km during the nighttime and 960 km during the day.

Sensor complement: [VEFI, CINDI (IVM, NWM), CORISS, DIDM, CERTO, PLP]

The sensor complement of C/NOFS permits observations of ionospheric irregularities in the equatorial F region. The overall objective is to determine when ionospheric structures will appear, over what spatial extent they will exist, and how severe the effects of such structures will become. 22)23)24)25)26)

The challenge is to effectively integrate these proven sensors into scintillation forecasts and nowcasts.

Figure 8: Schematic overview of the C/NOFS payload (image credit:UTD)

VEFI (Vector Electric Field Instrument):

VEFI is a NASA instrument designed and developed at GSFC (PI: Robert. F. Pfaff), contractor for data analysis: GST (Global Science & Technology, Inc.). The sensor has a long heritage, it was first flown on the Dynamics Explorer-2 mission (or Explorer 63) of NASA with a launch Aug. 3, 1981. The VEFI objective is to provide in-situ measurements of:

The C/NOFS VEFI experiment includes instrumentation to measure the DC and AC electric fields, it also features a burst memory, onboard signal processing, and a filter bank. In addition, the VEFI experiment includes a magnetometer sensor, a lightning detector (design and testing provided by the University of Washington), and a fixed-bias Langmuir probe which serves as the trigger input for the burst memory. 27)28)

CINDI is a NASA sponsored Mission of Opportunity within the SMEX program, a payload, developed by the University of Texas at Dallas (UTD), PI: R. A. Heelis. The CINDI investigation is carried out as an enhancement to the science objectives of C/NOFS. The objective is to investigate the role of ion-neutral interactions in the generation of small and large-scale electric fields in the Earth's upper atmosphere. Ion-neutral interactions are a key process in controlling the dynamics of all planetary atmospheres and their understanding is important to describing the electrodynamic connections between the sun and the upper atmosphere. 29)30)31)

CORISS is an occultation instrument of The Aerospace Corporation, El Segundo, CA. This instrument was formerly known as the Ionospheric Occultation Experiment (IOX) and was flown on PICOSat (launch Sept. 30, 2001) of DoD, the latter mission is also known as STP P97-1. IOX in turn is of TRSR (TurboRogue Space Receiver) heritage as flown on SUNSAT and Ørsted with improved codeless performance on 2 of the 8 channels. 32)

The CORISS objective is remote sensing of the electron density vertical profile at various bearings relative to track (where rising and setting GPS satellites are visible). The signals of the GPS constellation provide multi-point measurements of ionospheric irregularities and elicit the space/time variation of ionospheric turbulence. Limb profiles of TEC (Total Electron Content) can be inverted to produce vertical profiles of electron density.

Measurement output

Unit

Estimated
accuracy

Frequency
(cadence)

Estimated range of output values

Slant path TEC in a unit column

1016 ion/cm2

0.01 relative
3 absolute

0.1 Hz - 1.0 Hz
(times # of tracks)

0 - 1000

EPD (Electron Density Profile)

cm-3

Depends on ionospheric gradients

~22 / orbit (data collected at 1 Hz)

0 to 3 x 106

Onboard scintillation indices & spectra

S4: dimensionless
σφ: radians

S4: 0.05 & TBD

0.1 Hz
(times # of tracks)

S4: 0-1.5 &
σφ: 0-1
spectral slope: 0 to -3

Stratospheric temperature profile

K

1

~22 / orbit (data collected at 50 Hz)

0 - 300

High rate scintillation products

S4: dimensionless
σφ: radians

S4: 0.05 &
other: TBD

50 Hz for occulting satellites (burst mode)

S4: 0-1.5 &
σφ: 0-1
spectral slope: 0 to -3

Table 5: Overview of CORISS measurements

Note: S4 and σφ are scintillation parameters - where S4 is a measure of amplitude scintillation) and σφ is a measure of phase scintillation.

CORISS is a dual frequency GPS receiver. The nominal sampling time is 0.1 Hz for navigation data and 1 Hz for ionospheric occultation data (there is also a 50 Hz sampling mode to measure ionospheric scintillation as well as observables in the troposphere). The receiver performs phase measurements of both GPS frequencies L1 and L2, and code pseudoranges.

The CORISS instrument consists of four elements: a dual-frequency receiver, a S/C interface electronics module, a low noise amplifier/filter, and a dual-frequency GPS patch antenna. The instrument mass is about 3 kg, power is about 10 W, the data rate is 6-50 Mbyte/day if operated continuously.

The instrument is of STP -4 (Space Test Program-4) heritage with a launch in Oct. 1997, and of CHAMP heritage with a launch on July 15, 2000. DIDM is provided by AFRL, Hanscom Air Force Base, Bedford, MA, and built by Amptek Inc. of Bedford, MA. The objective of DIDM is to measure the Earth's electric field parallel to the magnetic field (in-situ measurements of the ion distribution and its moments within the ionosphere). DIDM measures magnitude and direction of the incoming ion flux. The electric field is derived from the relationship between electric field, measured ion drift velocity and measured magnetic field strength.

DIDM utilizes miniaturized state-of-the-art detector components and on-board digital signal processing. The instrument consists of two side-by-side ion detectors [MCP (Microchannel Plate)type] arranged within one unit of 2.2 kg mass. Both apertures are facing the S/C ram direction. Both detectors can measure the normal and perpendicular velocity components of incident ions. Ions entering through a pinhole aperture fall into a Retarding Potential Analyzer (RPA) cup. Those ions with energies higher than a certain threshold potential are electrostatically focused onto charge multiplying microchannel plates (MCPs) and high-resolution wedge and strip charge detecting anodes. The images created by these impacting ions on the anode are digitally processed on-board to reconstruct the full 3-D ion velocity vector. 33)

• The CERTO/TBB (Coherent Electromagnetic Radio Tomography/Triband Beacon Transmitter) package will also be flown on the "ROCSat-3 / COSMIC / FormoSat-3" mission constellation of six spacecraft with an expected launch in the fall of 2005.

• Furthermore, the CERTO/PLP package is also to be flown on the NPSat-1 (Naval Postgraduate School Satellite-1) mission, in turn a payload of the STP-1 (Space Test Program-1) multi-spacecraft mission of DoD with a planned launch in the fall of 2006. On the same mission is also a spacecraft called STPSat-1 (Space Test Program Satellite-1) with a sensor called CITRIS (Computerized Ionospheric Tomography Receiver In Space), a tri-frequency receiver also being developed by NRL. 34)35)

The CERTO objective is to determine ionospheric electron density by using a three-frequency beacon; cooperative ionospheric observations with fixed ground receivers. CERTO provides a global ionospheric map to aid the prediction of radio-wave scattering. This knowledge will improve navigation accuracy and communications capacity for military and commercial systems.

Figure 15: Illustration of the CERTO instrument (image credit: NRL)

PLP (Planar Langmuir Probe):

PLP is an instrument of AFRL/VSBX. PLP is operated in combination with DIDM. This device provides auxiliary data needed to interpret the ion drift measurements. Quantities that can be derived from the PLP sweeps are S/C potential, electron temperature, and density. The PLP voltage is swept for 1 s every 15 seconds typically between ±2.5 V in 32 steps. A selectable bias voltage can be added to account for the S/C potential. By interpreting the measured current/voltage characteristic the plasma parameters are being determined. The S/C floating potential is measured during the remaining 14 seconds of the measuring cycle. 36)

Ground segment:

The principal element of the ground segment of C/NOFS is SCINDA (Scintillation Network Decision Aid), a system of ground-based receivers (passive UHF / L-band /GPS scintillation receivers) which monitor the radio link to spacecraft in geosynchronous orbit. The stations are all located within 20 degrees of the Earth's magnetic equator. SCINDA consists of: 37)38)

• Passive UHF / L-band /GPS scintillation receivers

• Measures scintillation intensity, eastward drift velocity, and TEC

• Automated real-time data retrieval via internet.

The SCINDA tool is part of a process that results in simple tri-color maps of disturbances over the equator and the corresponding areas of likely communication outages. Such maps help scientists to better understand how scintillation structures develop, and enable operators to determine practical strategies for maintaining reliable communications. The data has been combined with the C/NOFS modeling and observations to produce now-casts and forecasts of scintillation regions.

16) Winfried Otto Schumann (May 20, 1888–September 22, 1974) was a German physicist (Technical University of Munich) who predicted the Schumann resonances mathematically, a series of low-frequency resonances caused by lightning discharges in the atmosphere.

The information compiled and edited in this article was provided byHerbert J. Kramer from his documentation of: "Observation of the Earth and Its Environment: Survey of Missions and Sensors" (Springer Verlag) as well as many other sources after the publication of the 4th edition in 2002. - Comments and corrections to this article are always welcome for further updates (herb.kramer@gmx.net).